Syrian government and Russian jets have stepped up the bombardment of a town in northern Syria held by the Islamic State of Iraq and the Levant (ISIL) group, increasing pressure on the fighters, according to a monitoring group.

Britain-based Syrian Observatory for Human Rights (SOHR) said the warplanes carried out at least 18 air strikes since dawn on Sunday on the town of Tabqa, just west of Raqqa, ISIL's de facto capital in Syria.

Separately, the activist group Raqqa Is Being Slaughtered Silently said on Facebook that at least six people died in the bombing.

SOHR, which relies on a network of activists inside Syria, said at least one person had died, with many more injured.

The bombing has prompted a mass exodus of locals from the town to safer areas.

Earlier this month, Syrian troops started an offensive aimed at cutting off Raqqa from the Turkish border.

Troops and militia members, backed by Syrian and Russian warplanes, pushed east from the government outpost of Ithriya, closing in on Tabqa.

The Kurdish-led Syrian Democratic Forces (SDF), backed by US air strikes, is also pushing towards Tabqa, located near Syria's largest dam, at the southern end of Lake Assad on the Euphrates.

The SDF, an alliance of Kurds and Arabs, seeks to retake the northern Syrian town of Manbij from ISIL, also known as ISIS, as part of their offensive in Raqqa province.

Manbij is strategically important as it controls a supply route from the Turkish border to Raqqa.

Clashes between SDF forces, backed by a US-led air power, and ISIL fighters raged on Sunday on the outskirts of Minbij, the SOHR said. No casualties were reported.

Manbij has been under ISIL control since 2014.

Border shootings

In another development on Sunday, Turkish military officials denied reports that its border guards killed at least 11 Syrians, mostly from one family, as they tried to cross into Turkey from northwestern Syria overnight.

The SOHR and several local activists said at least two women and four children were among those killed in the shootings as the refugees sought to cross into Turkey from the border village of Khirbet al-Jouz.

However, Reuters news agency quoted a Turkish military statement as stating: "Claims that Turkish soldiers killed nine people that were trying to cross the border in Hatay province ... are not true.

"Last night there was an attempt to cross the border illegally but no shots were fired directly on people.

"After warning shots, a group of seven to eight people ran towards the woods."

The SOHR, which tracks violence across Syria, say it has documented the deaths of nearly 60 civilians while trying to flee from Syria since the start of the year in shooting incidents by Turkish border guards.